Question

I wish to set an attribute on a public property in .NET, however I do not have access to the explicit property itself, as this has been code generated in another file.

I have this field:

public virtual string Name { get; set; } 

I wish to set this:

[ValidateNonEmpty('Name is required', ExecutionOrder = 1)] public virtual string Name { get; set; } 

My class is marked as partial, but you cannot have partial properties. I thought I was on to something with the MetadataType class which is a new feature of Dynamic Data and DataAnnotations, but alas I feel it can only be used with Dynamic Data, is this true?

Citations: http://blogs.oosterkamp.nl/blogs/jowen/archive/2008/10/16/metadatatype-attribute.aspx http://blogs.msdn.com/davidebb/archive/2008/06/16/dynamic-data-and-the-associated-metadata-class.aspx

Is there any way I can set this attributes (even through web.config!) without touching the code generated class?

Thanks in advance, Graham

Answer 1

This is a known nuisance; you simply can’t add metadata to the generated members.

There are 6 options here (in order of increasing effort):

• if you own the attribute, make it possible to declare it against the class, for example: [ValidateNonEmpty('Name', 'Name is required', ExecutionOrder = 1)] – then add multiple attributes to the partial class definition
• use a virtual / interface / etc method to query this, rather than via attributes
• sublass the generated type; override or re-declare the member, adding the metadata (really messy)
• use a custom TypeDescriptionProvider to provide dynamic metadata (lots and lots of work) – assuming that the consumer respects TypeDescriptor; most binding-related consumers do, but for example, Expression (used by many LINQ providers) doesn’t
• change the code-generator / write your own
• try to extend something like PostSharp to do the work (I haven’t found a way to do this, but I’ve love to hear if you find a way!)

I usually have success with the first option, unless it is a system-defined attribute ([DisplayName], etc). If [ValidateNonEmpty] is defined by dynamic data, then you might not be able to do this.